Nonwovens, Method for the Production Thereof and the Use Thereof

ABSTRACT

The invention relates to a method for the production of nonwovens, in which a solution of cellulose carbamate in N-methylmorpholine-N-oxide is forced through the holes of a nozzle beam and is stretched by a multiple by means of an airflow. The fibre curtain which is formed is laid down on a perforated conveyer belt, washed and dried. The invention likewise relates to nonwovens of this type and the use thereof.

The invention relates to a method for the production of nonwovens, in which a solution of cellulose carbamate in N-methylmorpholine-N-oxide is forced through the holes of a nozzle beam and is stretched by a multiple by means of an airflow. The fibre curtain which is formed is laid down on a perforated conveyer belt, washed and dried. The invention likewise relates to nonwovens of this type and the use thereof.

Nonwovens are textile fabrics in which the fabric formation is effected not by weaving or stretching but by interlocking of the fibres with subsequent fixing. Because of the versatile possibilities for use and the comparatively low production cost in comparison with knitted and woven fabrics, nonwovens still have high annual growth rates.

The advantages of these nonwoven materials reside in a high moisture absorption, the production methods allow high variability in density and thickness and also lead to an extensive surface isotropy. As a result of these advantageous properties there are numerous possibilities for use in medicine for hygiene products, e.g. operating theatre covers, bed sheets, wound coverings, gauzes etc., as cloths of all types in the household and as decorative nonwovens, e.g. tablecloths, serviettes, as lining fleeces in the clothing industry and also for technical applications, e.g. insulation mats, cover mats.

In principle, fleece formation from short fibres, staple fibres or continuous filaments is possible. Formation of the web for cut fibres is effected according to the dry or also wet fleece method. In the case of the former, the staple fibres which are delivered in bale form are combed to form a fleece by means of a comb or card and are placed on a belt. During the wet fleece method, formation of the web is effected by applying a suspension of fibre and water onto a perforated screen belt with subsequent pressing out of the water. The fleeces formed can then be further consolidated by needle felting, thermal and/or chemical treatment or even by means of a water jet.

In the production of spun nonwovens, direct combination of spinning and fleece formation process is effected. Both melt and dry spinning methods and wet spinning methods are suitable for the fleece formation based on filaments. A large number of fibre-forming polymers is known as starting material for the nonwovens. Nonwovens made of continuous materials are produced preferably from polyester and polypropylene (melt spinning—meltblown nonwovens) and also polyacrylonitrile (wet spinning—spunlaid nonwovens).

Viscose methods, i.e. regenerated cellulose fibres, are used preferably as short or staple fibres for nonwoven production. Nonwovens based on regenerated cellulose fibre have a significant proportion of the market with anticipated continuous growth because of their property potential.

Since the viscose method, according to which still the greatest part of regenerated cellulose fibres is produced, involves significant environmental pollution, e.g. carbon disulphide, hydrogen sulphide, heavy metals and high investment costs, for years efforts have been made to replace the viscose method by alternative methods. Methods based on direct dissolving of pulp in a suitable solvent or alternative derivatisation variants without carbon disulphide have been developed. These activities extend also to the production of nonwovens made of cellulose.

The so-called “Bemliese” method, in which cotton linters are formed into the fleece according to the cuprammonium method, is one of the first spun fleece methods proposed for the production of cellulose nonwovens from continuous filaments (U.S. Pat. No. 3,833,438).

The formation of melts of synthetic polymers by extrusion of the melt through fine nozzle holes into a gas flow which flows in the fibre direction, by means of which the fibre is cooled and thereby drawn and stretched to a multiple of its length, is a method for nonwoven production which has been known for a fairly long time. This process which is also termed “meltblown” is suitable for the production of microfibres. In the European Patent specification EP 0 920 548 B1, the application of meltblowing also for solutions of cellulose in amine oxides, preferably N-methylmorpholine-N-oxide (NMMO) is claimed. The method is influenced greatly by the viscosity of the cellulose/NMMO solution, this is in turn determined by the polymer (cellulose), the molecular weight (DP of the cellulose) and also the concentration.

A further known, environmentally friendly method for the production of cellulose fibres and moulded articles is based on the precipitation of cellulose carbamate from a solution of this cellulose derivative in sodium hydroxide solution (EP 57 105, EP 178 292). Cellulose carbamate is formed during the conversion of cellulose with urea at increased temperature and can be dissolved in cold diluted sodium hydroxide solution.

The low solid concentration of the cellulose in the spinning solution which generally is 8 to 12% is disadvantageous for the productivity of all cellulose wet spinning methods, in particular also that of NMMO and the carbamate method.

Starting herefrom, it was the object of the present invention to provide a method for the production of nonwovens made of cellulose carbamate, which, with good product properties, fulfils the demands with respect to low investment and production costs and also low environmental impact. It was also the object of the present invention to provide nonwovens with superior product properties.

This object is achieved by the method having the features of claim 1 and the nonwovens having the features of claim 20. The further dependent claims reveal advantageous developments. In claims 26 to 30, uses of the nonwoven according to the invention are indicated.

According to the invention, this object is achieved by a meltblowing method, in which a spinning solution is forced through a nozzle block at increased temperature, the fibres emerging from the nozzle holes are drawn through a temperature-controlled airflow and are thereby stretched and cooled and are coagulated by spraying with water. Subsequently, the fibres are laid down on a conveying device whilst forming a random orientation, the individual fibres being self-binding. Subsequently, the solvent is removed by washing.

It is thereby a particular feature of the method that a solution of cellulose carbamate in N-methylmorpholine-N-oxide is used as spinning solution.

Solutions of cellulose carbamate in NMMO can be formed comparably to corresponding cellulose solutions. The advantages of the cellulose carbamate in comparison with cellulose reside in improved solubility in NMMO so that the process can take place at higher concentrations. The result thereof is a higher throughput and lower costs in recycling the solvents.

It was now found surprisingly that, from solutions of cellulose carbamate in NMMO, nonwovens can be produced according to the meltblowing method with higher strength and higher water absorption compared with cellulose, which makes these fleeces particularly suitable for use in hygiene articles for medicine, for household articles but also as decorative nonwovens.

The nozzle block which is used preferably has at least 20, preferably 150 to 1500 and particularly preferred 500 to 1000, nozzle holes per meter of nozzle width. With respect to the diameter of the nozzle holes, diameters of 0.05 to 1 mm and particularly from 0.3 to 0.5 mm are thereby preferred.

The spinning solution temperature during extrusion is preferably in the range between 80 and 130° C., particularly preferred in the range of 90 to 110° C. The spinning solution conveying quantity is thereby between 0.25 to 2.5, particularly preferred 0.75 to 1.5 g per nozzle hole and minute.

The airflow used for withdrawing the fibres from the nozzle is preferably heated to 100 to 190° C., particularly preferably an air temperature of 120 to 160° C. should be set in the nozzle interior. The pressure produced by the airflow in the nozzle interior is preferably at 0.03 to 1.5 bar, particularly preferred between 0.15 and 0.8 bar.

The nozzle output speed is preferably 1 to 20 m/min, particularly preferred however 3 to 10 m/min. As a result of the hot airflow present, the extruded threads are stretched preferably to 50 to 800 times their length after leaving the nozzle, which leads to spinning speeds of up to 10000 m/min.

Single fibre diameters significantly less than 10 μm and single fibre titres of less than 1 dtex can be achieved by corresponding adjustment of the stretching conditions.

At least 10 cm below the nozzle, the forming fibre web is sprayed with water or with an NMMO/water mixture with an NMMO content of preferably at most 20%, preferably 5 to 15%, and thus is coagulated.

The fibre web is laid down at high speed on a perforated belt and the result is consequently interlocking of the fibre and a fleece is formed which has such good mechanical properties that the subsequent operations, such as washing with a water jet at high pressure, pressing out and drying can be implemented without additional consolidation of the fleece.

In a preferred embodiment of the method; the spinning solution contains from 5 to 20% by weight of cellulose carbamate. A further preferred variant provides that the spinning solution contains 20 to 30% by weight of cellulose carbamate. In the latter case, a solution of cellulose carbamate in NMMO is thus obtained which has surprisingly a lyotropic behaviour, i.e. the cellulose carbamate is present in a liquid crystalline state. The particularly advantageous application is then produced herefrom that the molecules during spinning are orientated virtually perfectly in the fibre direction as a result of the shearing in the nozzle channel, as a result of which the fibres have an outstandingly high orientation and hence very great strength.

According to the invention, a cellulose carbamate nonwoven is likewise provided from a random orientation of continuous filaments or from fibres with a finite length which are produced by tearing individual capillaries, said nonwoven being able to be produced according to the method according to one of the claims 1 to 17.

The nonwovens produced according to the claimed method preferably have basis weights between 20 and 500 g, preferably have a water retention capacity of at least 180% and, after conditioning, strengths in the longitudinal direction of at least 350 N/m and in the transverse direction of at least 120 N/m with a basis weight of 33 g/m².

The nonwovens according to the invention are used preferably in medicine, in particular as operating theatre covers, bed sheets, wound coverings, gauzes or cotton wool pads. Likewise the nonwovens can be used also as hygiene materials or cloths in the household. A further field of application of the nonwovens according to the invention is decorative nonwovens, in particular table cloths, serviettes or curtains and also lining fleeces in the clothing industry.

If the cellulose carbamate is used in a quantity which makes possible a lyotropic solution as described above, it is likewise possible to use the nonwovens according to the invention as reinforcing material.

With reference to the subsequent examples, the subject according to the invention is intended to be explained in more detail without restricting the latter to the embodiment variants described herein.

EXAMPLE 1

180 g cellulose carbamate (DP-Cuen 453) are mixed in a kneader with 2430 g of a 50% aqueous NMMO solution, this solution is concentrated by distilling off water under a vacuum of 80 mbar into NMMO monohydrate and the cellulose carbamate is thereby dissolved. The spinning solution formed has a cellulose carbamate content of 12%. The spinning solution with a temperature of 90° C. is filtered via a 50 μm filter and forced by means of a spinning pump through a 15 cm wide nozzle with 60 nozzle holes of the diameter 0.48 mm with a conveying quantity of 0.9 g/hole/min. The fibres emerging from the nozzle holes at a speed of 4 m/min are stretched to 459 times their length by means of air heated to 125° C. flowing past, are sprayed with a mixture comprising 7.5% NMMO/92.5% water approx. 20 cm below the nozzle and laid down on a conveyer belt with a speed of 1 m/min. The formed fleece is washed with a vigorous water jet, squeezed out and dried under tension whilst allowing a shrinkage of 12%.

Nonwoven Parameters:

Basis weight: 36 g/m² Strength longitudinally (conditioned): 356 N/m Strength transversely (conditioned): 120 N/m Water retention capacity (DIN 53814): 192%

EXAMPLE 2 Comparative Example

150 g PHK pulp (DP-Cuen 840) are mixed in a kneader with 1945 g of a 50% aqueous NMMO solution, this solution is concentrated by distilling off water under a vacuum of 80 mbar to form NMMO monohydrate and the pulp is thereby dissolved. The spinning solution formed has a cellulose content of 8%. The spinning solution with a temperature of 90° C. is filtered via a 50 μm filter and forced by means of a spinning pump through a 15 cm wide nozzle with 60 nozzle holes of the diameter 0.48 mm with a conveying quantity of 0.9 g/hole/min. The fibres emerging from the nozzle holes at a speed of 4 m/min are stretched to 459 times their length by means of air heated to 125° C. flowing past, are sprayed with a mixture comprising 7.5% NMMO/92.5% water approx. 20 cm below the nozzle and laid down on a conveyer belt with a speed of 1 m/min. The formed fleece is washed with a vigorous water jet, squeezed out and dried under tension whilst allowing a shrinkage of 12%.

The fleece formed from regenerated cellulose had the following parameters.

Basis weight: 37 g/m² Strength longitudinally (conditioned): 133 N/m Strength transversely (conditioned): 45 N/m Water retention capacity (DIN 53814): 145% 

1. Method for the production of nonwovens according to the meltblowing method, in which a spinning solution is forced through a nozzle block at increased temperature, the fibres emerging from the nozzle block are subjected to a flow of temperature-controlled airflow, the result being stretching of the fibres with subsequent cooling, the threads are coagulated by spraying with water, the fibres are placed on a conveying device whilst forming a random orientation and the solvent is removed by washing, characterised in that a solution of cellulose carbamate in N-methylmorpholine-N-oxide is used as spinning solution.
 2. Method according to claim 1, characterised in that the nozzle block has at least 20, in particular from 150 to 1500, nozzle holes per meter.
 3. Method according to claim 1, characterised in that the diameter of the nozzle holes is from 0.05 to 1 mm.
 4. Method according to claim 1, characterised in that the spinning solution temperature in the nozzle is 80 to 130° C.
 5. Method according to claim 1, characterised in that the spinning solution conveying quantity is from 0.25 to 1.5 g per nozzle hole and minute.
 6. Method according to claim 1, characterised in that the airflow has a temperature of 100 to 190° C.
 7. Method according to claim 1, characterised in that a pressure of 0.03 to 1.5 bar is set by the airflow in the nozzle.
 8. Method according to claim 1, characterised in that the nozzle output speed of the fibres is from 1 to 20 m/min.
 9. Method according to claim 1, characterised in that the threads are stretched by the factor 50 to 800 by means of the airflow.
 10. Method according to claim 1, characterised in that the spinning speed is in the range of 150 to 10000 m/min.
 11. Method according to claim 1, characterised in that the fibres have a diameter of at most 20 μm.
 12. Method according to claim 1, characterised in that the fibres have a fineness of at most 2.5 dtex.
 13. Method according to claim 1, characterised in that the fibres are sprayed at least 10 cm below the nozzle with water or with a mixture comprising N-methylmorpholine-N-oxide and water.
 14. Method according to claim 13, characterised in that the mixture contains from 0.5 to 20% by weight of N-methylmorpholine-N-oxide.
 15. Method according to claim 1, characterised in that the fibres are laid down on a perforated conveyer belt.
 16. Method according to claim 1, characterised in that the washing is effected by a water jet at high pressure.
 17. Method according to claim 1, characterised in that the nonwoven is pressed out and/or dried after washing.
 18. Method according to claim 1, characterised in that the spinning solution contains from 5 to 20% by weight of cellulose carbamate.
 19. Method according to claim 1, characterised in that the spinning solution contains 20 to 30% by weight of cellulose carbamate.
 20. Nonwoven comprising a random orientation of fibres made of cellulose carbamate, producible according to the method according to claim
 1. 21. Nonwoven according to claim 20, characterised in that the nonwoven has a basis weight of 20 to 500 g/m².
 22. Nonwoven according to claim 20, characterised in that the nonwoven has a water retention capacity of at least 180%.
 23. Nonwoven according to claim 20, characterised in that the nonwoven has a strength in the longitudinal direction of at least 250 N/m.
 24. Nonwoven according to claim 20, characterised in that the nonwoven has a strength in the transverse direction of at least 80 N/m.
 25. Nonwoven according to claim 20, characterised in that the cellulose carbamate has a substitution degree DS of 0.1 to 1.5.
 26. Use of the nonwoven according to claim 20 in medicine, in particular as operating theatre covers, bed sheets, wound coverings, gauzes or cotton wool pads.
 27. Use of the nonwoven according to claim 20 as hygiene materials or cloths in the household.
 28. Use of the nonwoven according to claim 20 as decorative nonwovens, in particular table cloths, serviettes or curtains.
 29. Use of the nonwoven according to claim 20 as lining fleeces in the clothing industry.
 30. Use of the nonwoven according to claim 20 as reinforcing material. 